The present invention relates to a new memory checker for checking reading and writing data in a memory module, and more particularly to such a memory checker which occupies less memory bits to effectively check reading and writing data in a memory module. Conventionally, a big memory space must be reserved in the memory module for storing checking bits corresponding to the respective data byte address. The memory checker of the invention greatly improves the demand for memory capacity, and it immediately sends any detected error information to the system, as the system is reading the data, so that the system knows quickly whenever there is wrong data stored in the memory module.